use std::ops::ControlFlow;
fn each_step<B>(lo: i64, hi: i64, step: i64,
                mut f: impl FnMut(i64) -> ControlFlow<B>) -> ControlFlow<B> {
    let mut i = lo;
    while i < hi { f(i)?; i += step; }
    ControlFlow::Continue(())
}
fn first_multiple_of_7(lo: i64, hi: i64) -> i64 {
    match each_step(lo, hi, 3, |it| {
        if it.rem_euclid(7) == 0 { ControlFlow::Break(it) }
        else { ControlFlow::Continue(()) }
    }) {
        ControlFlow::Break(value) => value,
        ControlFlow::Continue(()) => -1,
    }
}

fn main() {assert_eq!(first_multiple_of_7(1,25),7);assert_eq!(first_multiple_of_7(2,6),-1);assert_eq!(first_multiple_of_7(0,1),0);assert_eq!(first_multiple_of_7(-14,0),-14);}